An apparatus and method for controlling hydraulic flow, mixing and gas transfer characteristics in long vertical shaft bioreactors.
This invention relates to long vertical shaft bioreactors for the aerobic biological treatment of wastewater and aerobic digestion of biodegradable sludges. In particular, the invention relates to an apparatus and method to improve control of hydraulic flow, inter-zonal mixing and gas transfer characteristics in such bioreactors.
Long vertical shaft bioreactor systems are well known in the prior art. For example, U.S. Pat. Nos. 5,645,726 and 5,650,070, Pollock, which issued on Jul. 8, 1997 and Jul. 22, 1997 respectively, relate to bioreactors adapted for the treatment of biodegradable sludge and wastewater. These bioreactors comprise a circulatory system that includes at least two long substantially vertical side-by-side or coaxial chambers (i.e. a downflow and an upflow chamber) in communication with each other at their upper and lower ends. In particular, the upper ends of the chambers are connected through a surface basin and the lower ends communicate in a common mix zone located immediately below the lower end of the downflow chamber. A xe2x80x9cplug flowxe2x80x9d zone with no recycle is located immediately below, and communicates with, the mix zone. As used in this patent application xe2x80x9cplug flowxe2x80x9d refers to net downward migration of solid particles from the mix zone toward an effluent outlet located at the lower end of the reactor. The net downward migration may include some local back mixing.
The wastewater or sludge waste to be treated is caused to circulate repeatedly through and between the downflow and the upflow chambers, the surface basin and the mix zone. A portion of the circulating flow is directed to the plug flow zone and is removed at the lower end thereof as effluent.
Normally the waste-containing liquor comprising biomass is and microbes, referred to as xe2x80x9cmixed liquorxe2x80x9d, is driven through the circulatory system by the injection of an oxygen-containing gas, usually air, into either or both of the mix zone and the plug flow zone. Typically, in a reactor for the treatment of wastewater, the air is injected 5-10 feet above the bottom of the reactor and, optionally, a portion of air is also injected immediately below the lower end of the downflow chamber. The deepest air injection point divides the plug flow zone into a quasi plug flow zone with localized back mixing above the deepest point of air injection, and a strict plug flow zone with no mixing below the deepest point of air injection. The influent wastewater is introduced into the upflow chamber a short distance above the lower end of the downflow chamber. At start-up, air is injected at depth through the influent line into the upflow chamber thus causing liquor circulation between and through the upflow and downflow chambers in the nature of an air lift pump. Once circulation has started, all the air injection is diverted to the mix zone and/or plug flow zone. Bubbles rising out of these zones are entrained in the upflow chamber and excluded from the downflow chamber (because the downward flow of liquor in the downflow chamber exceeds the rise rate of the bubbles). Thus all the air bubbles are transferred to the upflow chamber and stable circulation is maintained.
Usually the surface basin is fitted with a horizontal baffle at the top of the upflow chamber to force the mixed liquor to traverse a major part of the basin and release spent gas before again entering the downflow chamber for further treatment. A zone of turbulence is created at the lower end of the downflow chamber by the turn-around velocity head as the circulating flow reverses from downward to upward flow. This mix zone is not well defined but typically is between 15-25 feet deep. A portion of the mixed liquor in the mix zone flows downwardly into the top of the plug flow zone in response to an equal amount of treated effluent being removed from the lower end of the plug flow zone into an effluent line as discussed above.
Reaction between waste, dissolved oxygen, nutrients and biomass (including an active microbial population), substantially takes place in an upper circulating zone of the bioreactor defined by the surface basin, the upflow and downflow chambers and the mix zone. The majority of the contents of the mix zone circulate upwardly into the upflow chamber. In this upflow chamber undissolved gas, mostly nitrogen, expands to help provide the gas lift necessary to drive circulation of the liquor in the upper part of the reactor. The spent gas is released from the liquor as it traverses the horizontal baffle in the surface basin. The plug flow zone located below the upper circulating zone provides a final treatment or xe2x80x9cpolishxe2x80x9d to the mixed liquor flowing downward from the mix zone to effluent extraction at the lower end of the reactor. The injected oxygen-containing gas dissolves readily under pressure in the liquor in the plug flow zone where there is localized back mixing resulting in a slow net downward movement of liquor. Undissolved gas (bubbles) migrate upward to the very turbulent mix zone under pressure. The gas to liquid transfer in this zone is very high reaching overall reactor oxygen transfer efficiencies in excess of 65%. The products of the reaction are carbon dioxide and additional biomass which, in combination with unreacted solid material present in the influent wastewater, forms a sludge (or biosolids).
Long vertical shaft bioreactors designed for aerobic treatment of wastewater and sludge are generally similar. However, wastewater treatment bioreactors typically require a much smaller plug flow zone. Additionally, sludge treatment bioreactors preferably include two different aeration distributors for injecting air into the reaction vessel at two separate locations, namely in both the mix zone and the plug flow zone as described above.
The principal products of aerobic digestion of sludge biosolids in the mesophilic temperature range (up to approximately 40xc2x0 C.) are carbon dioxide, nitrate nitrogen, and reduced sludge mass. The principal products of aerobic digestion in the thermophillic temperature range (approximately 45xc2x0 C.-70xc2x0 C.) are carbon dioxide and ammonia.
While existing long vertical shaft bioreactors, such as those described in U.S. Pat. Nos. 5,645,726 and 5,650,070, are useful in the treatment of wastewater and sludge biosolids, they exhibit several shortcomings which limit their commercial effectiveness. When such prior art bioreactors are designed to accommodate a wide range of loads and flows, the mix and plug flow zones may become over sized resulting in a loss in hydraulic and oxygen transfer efficiency under some operating conditions. As a compromise, prior art bioreactors are typically optimized for one condition - usually average load and flow. Unfortunately average conditions only occur briefly two or three times a day in a typical municipal waste treatment plant operating under diurnal loading conditions.
Under increasing loads and flows, where the air rate must be increased to satisfy the greater biological air requirement, the efficiency of the reactor is compromised. This is due to the increase in the circulation rate resulting from the increase in air rate. Increasing the circulation rate actually lowers the dissolved gas concentration, lowers the respiration rate of the microbes and increases hydraulic head losses, as explained in further detail below.
Hydraulic Considerations
When the circulation velocity in the upper circulating zone of the bioreactor increases, the mixing time at maximum pressure for the air and water decreases. Furthermore, for any given air rate, increasing the liquor velocity and therefore the liquor volume flowing past the point of air injection, dilutes the concentration of available air per unit volume of liquor. This reduces the saturation potential of air in water. Normally the downflow chamber is approximately one quarter the cross-sectional area of the reactor body so that an increase in liquor flow velocity in the upflow chamber, caused by the increased air rate, has nearly four times the impact on velocity of the liquor in the downflow chamber. The increased air rate will continue to increase the circulation velocity until hydraulic head equilibrium is established (i.e. when the hydraulic resistance associated with the downflow chamber balances the gas lift effect in the upflow chamber created by the additional air). In large reactors where the hydraulic loss in big pipes is relatively small, the liquor velocities in the downflow chamber can be 10-15 ft/sec. Experiments have shown the flow of liquor through the downflow chamber penetrates the plug flow zone about 1-1.5 ft. for each ft/second of downward velocity. Very high air rates that create downward flow rates of 10-15 ft./second can effectively eliminate the function of the plug flow zone, in wastewater treatment bioreactors, by mixing most of the plug flow volume into the recirculating flow. These same phenomena would also reduce the plug flow zone in bioreactors adapted for treatment of sludge biosolids by about 15-20%.
Accordingly, high downflow chamber flow rates are to be avoided because hydraulic losses in the downflow chamber portion of the bioreactor add directly to the head loss in the effluent line (the effluent line can be thought of hydraulically as a continuation of the downflow chamber, mix zone and plug flow zone). Overcoming such head loss requires pressurizing the effluent flow and/or pumping the influent flow into the reactor, resulting in operational inefficiencies.
Inter-zonal Mixing Considerations
Tracer studies on the plug flow zone of a bioreactor adapted for treatment of sludge biosolids (such as the VERTADTM(trademark) bioreactor described in U.S. Pat. No. 5,650,070) show that, although the 10-15 ft./second circulating hydraulic flow penetration into the plug flow zone is substantially arrested 15-25 feet below the lower end of the downflow chamber, there remains a minor movement or flow below the zone of penetration. This flow moves very slowly and behaves as a locally backmixed xe2x80x9cfrontxe2x80x9d. The xe2x80x9cfrontxe2x80x9d moves downward at about 1-3 feet per minute in clean water and about 0.5-1.5 feet per minute in 4% sludge. This liquid flow downward is approximately equivalent to the compressed volume flow of air bubbles moving up through the plug flow section of the reactor. High air rates resulting in higher mixing rates allow this xe2x80x9cfrontxe2x80x9d to proceed through the plug flow zone even more quickly thus reducing the effectiveness and value of the plug flow zone.
Biological considerations.
It is well known that the rate of biological oxygen demand (BOD) removal in bio-oxidation is a function of the BOD concentration up to a maximum rate. For any acclimatized biomass (biological mass of microbes) there is a concentration of BOD beyond which there will be no increase in removal rate. When the liquor circulation rate increases, the BOD concentration at the point of influent injection is diluted and the respiration rate drops from the desirable maximum value to a much lower value, thus reducing the reactor""s biological capacity to bio-oxidize the organics in the wastewater or sludge.
Maintaining a higher respiration rate in the main circulating zone of the bioreactor results in removal of most of the BOD in the circulating zone. This allows the plug flow zone to operate at a lower respiration rate which in turn has the effect of preserving more of the dissolved air for use in flotation separation (i.e. separation of the biomass in the surface separation basin of the bioreactor). Accordingly, a reduction in the respiration rate in the circulating zone due to an increase in the air injection rate may reduce the dissolved air available for subsequent flotation separation.
In summary, increasing the air rate to accommodate increases in load or flow on an already optimized long vertical shaft bioreactor will have the following negative effects:
1. Lower dissolved oxygen levels.
2. Reduced respiration rate.
3. Increased hydraulic losses in the effluent line.
4. Partial mixing of the plug flow zone.
5. Reduced dissolved air for subsequent flotation separation.
The present invention not only offsets these negative impacts but can be adjusted on-line to improve the performance of the bioreactor for any load and flow.
In accordance with the invention, an aerobic bioreactor having elongate upflow and downflow chambers which are in fluid communication at their upper and lower ends is provided. The bioreactor further includes an influent conduit for discharging biodegradable waste into the upflow chamber; an effluent conduit for extracting effluent from a lower portion of the bioreactor; and a gas inlet for injecting oxygen-containing gas into the bioreactor to drive the circulation of a liquor comprising the biodegradable waste between the upflow and downflow chambers. The invention is characterized in that the bioreactor further includes a flow control device mounted in the upflow chamber upstream from a discharge end of the influent conduit, wherein the flow control device adjustably regulates the circulation velocity of the liquor.
Preferably the circulating liquor changes flow direction from downward flow to upward flow in a turbulent mix zone of the bioreactor located proximate the lower end of the downflow chamber and the flow control device further includes:
(a) an upper plate mounted in the upflow chamber near the upper end of the mix zone, the upper plate having a plurality of apertures therein permitting passage of the circulating liquor therethrough; and
(b) a lower plate mounted in the bioreactor beneath the upper plate near the lower end of the mix zone, the lower plate providing a partial barrier to flow of the liquor between the mix zone and a plug flow zone located below the mix zone.
The flow control device may also include a plurality of spaced-apart flow diverting plates extending vertically between the upper and lower plates and an impingement plate in the mix zone above the lower plate for diverting downwardly flowing liquor toward the lower plate.
In use, the flow control device imposes a hydraulic head loss near the lower end of the upflow chamber, whereby the circulating liquor on the lower side of the upper plate in communication with the effluent conduit is maintained at a higher pressure than the circulating liquor on the upper side of the upper plate in communication with the influent conduit, thereby causing the biodegradable waste to flow into the bioreactor through the influent conduit, and the effluent to flow out of said bioreactor through the effluent conduit, without the use of pumps.
The invention also encompasses a method of improving the efficiency of long vertical shaft aerobic bioreactors having elongate upflow and downflow chambers which are in fluid communication at their upper and lower ends; an influent conduit for discharging biodegradable waste into the upflow chamber; an effluent conduit for extracting effluent from a lower portion of the bioreactor; and a gas inlet for injecting oxygen-containing gas into the bioreactor to drive the circulation of a liquor comprising the biodegradable waste between the upflow and downflow chambers. The method includes the steps of:
(a) adjusting the volume of said oxygen-containing gas injected into the bioreactor in response to changes in the volume and/or concentration of biodegradable waste discharged into the bioreactor, thereby optimizing the rate of aerobic digestion of the waste; and
(b) reducing the circulation velocity of the liquor to increase the residence time of the liquor in the upflow chamber.